Motor vehicles emit a complex mixture of toxic chemicals that contributes substantially to the burden of air pollution exposure in this country. While a consistent link between ambient air pollution and cardiopulmonary effects has been demonstrated, very little knowledge exists regarding exposures and health effects of traffic- related air pollution specifically during the commute. Even in communities with low background levels of air pollution, there are intense, short-term exposures during commutes that may contribute to adverse health effects. Healthy, exercising populations (such as cyclists) have been neglected in previous air pollution research. Such populations with activities that increase lung dose are a potentially vulnerable subgroup among commuters. Therefore, a need exists to improve our understanding of the interactions between sources, exposures, and health effects of traffic-related air pollution. The objective of this research is to apply innovative modeling and exposure assessment and modeling techniques to improve our understanding of the impact of traffic-related air pollution on human exposure and health. To achieve this objective we propose the following specific aims: (1) Conduct a two-year study of commuters' personal exposures to traffic-related air pollution among different modes (car, bike) and routes (high- and low-traffic); (2) Develop and evaluate an exposure modeling system that integrates mechanistic air pollution fate and transport modeling, path-following exposure estimation, and Bayesian updating; and (3) Evaluate the short-term association of personal exposure levels during morning commutes with subclinical respiratory and cardiovascular responses that are central to the hypothesized biologic pathway linking air pollution with cardiovascular events. We hypothesize that personal exposures to traffic-related air pollution will be significantly higher for cyclists (vs. car drivers) and on high-traffic routes (vs. low-traffic), and that our model system will successfully predict average cumulative exposures for commuters traveling different route/mode combinations. We also hypothesize that increased personal exposure to traffic-related air pollution experienced during a commute will be associated with acute increases (from pre- to post-commute) in cardiovascular and respiratory responses. This project addresses knowledge gaps on the exposures and related health effects of a source of air pollution that affects the majority of the U.S. population. Given the emerging evidence on the adverse effects of traffic- related air pollution, information gained from this study can empower citizens to reduce their daily exposures to traffic-related air pollution. The daily commute, whether by bicycle, motor vehicle, or other mode, is an experience shared by nearly all Americans. Although ambient air pollution is generally considered a ubiquitous, involuntary hazard, research that develops new knowledge to reduce exposures to traffic-related air pollution during commuting has the potential to produce a substantial public health impact.